The present invention relates to systems for measuring light spectra, for example, for absorption, transmission, or reflection spectroscopy, and in particular to an optical spectrometer providing compact and robust construction.
Optical spectrometers provide a measurement of light intensity over multiple frequencies. An optical spectrometer may measure the spectrum of an unknown light source or be used with a known light source to measure absorption of a material when light from the known light source passes through or is reflected from the measured material before being detected. This latter absorption spectrum is derived by subtracting the spectrum of the light received at the detector from the spectrum of the known light source.
Distinguishing the intensity of different frequencies of light, as needed for determining a spectrum, is normally accomplished by using an analyzing filter system and a broadband light detector, the latter which does not distinguish among frequencies and is ideally equally sensitive to all frequencies of interest. The analyzing filter system is changed as a function of time so that the broadband light detector receives different frequencies of light as a function of time. In this way the time varying signal from the broadband detector may be decoded into a spectrum providing the intensity of different frequencies of light.
For determining the spectrum of an unknown light source, the analyzing filter system may be applied directly to the light source. For absorption spectroscopy, the analyzing filter system may be placed after a sample to be analyzed, to receive reflected or absorbed light from the sample as illuminated by a known light source. This light is then passed to the detector. Alternatively, the analyzing filter system may be placed in front of the sample to modify the light from a known light source before it is reflected or absorbed by the sample and then received by the broadband detector.
Conventional optical spectrometers may use a frequency dispersive element as an analyzing filter system, such as a diffraction grating. The angle of incidence of the light on the diffraction grating may be changed to generate a series of narrowband monochromatic beams each approximating a single spectral line. Independent measurements of the different monochromatic beams by the detector allow a full spectrum to be assembled. Generally, a slit may be used to successively isolate each narrowband monochromatic beam for sequential measurement by a single detector, or the monochromatic beams may be measured in parallel by a multi-detector array.
Fourier transform spectrometers may use an interferometer as an analyzing filter assembly to produce a light beam having a multi-frequency spectrum approximating a periodic sinusoid with regular zero values for frequencies within the range of interest. This sinusoidal spectrum is generated by reflecting a broadband light beam back on itself so that the light is subject to constructive and destructive interference at different frequencies. The period of the interference may be changed, for example, by moving a mirror of an interferometer, so that the sinusoid of the spectrum is modulated. Generally, higher optical frequencies will have a higher rate of modulation so that measurements made with the broadband detector may be processed by the Fourier transform to reveal a spectrum.
Both of these types of spectrometers require relatively large optical paths for good resolution and may further require complex precision machinery to move optical elements during the measurement process. As a result, low-cost and compact spectrometers, potentially useful in a variety of applications, are difficult to produce.